To elucidate the conformation and interactions of proteins in solution nuclear magnetic resonance (NMR) spectroscopy is the method of choice. To study strong and selective peptide-protein binding, as a model for hormone peptide-receptor interactions the ribonuclease S system has been chosen, since it provides a simple enzymatic test of biological viability. Selectively, 13C-enriched peptides of the (1-15) amino-terminal portion of ribonuclease were synthesized, and resolved resonances of individual 13C atoms in the peptide-protein complex were monitored. Selective 13C enrichment of methionine residues in cytochrome c have enabled us to monitor the conformational transitions of the protein and to show the existence of two distinct stable forms. Both at natural abundance (1.1%) and with 13C enrichment the 13C NMR technique has been extended to the observation of Met, His, Ala, Asp and Tyr residues in several proteins, and to the sugar component of glycoproteins.